Semiconductor devices are developed to have higher integration, higher speed and lower power consumption, which limit the application of bulk silicon substrates more and more. On the contrary, Silicon-On-Insulator (SOI) substrates can realize dielectric isolation of components in an integrated circuit, avoid a latch-up effect in a bulk silicon Complementary Metal Oxide Semiconductor (CMOS) circuit, have the advantages of small parasitic capacitance, high integration, high speed, simple process and small short-channel effect, and are suitable to be used in low-power-consumption low-voltage circuits. Therefore, SOI substrates are used more and more widely to form semiconductor devices. In a radio frequency (RF) device, a SOI substrate may be employed to reduce parasitic capacitance of the substrate. Besides, the SOI substrate may improve high-frequency characteristics and operational speed of the RF device.
Referring to FIG. 1, FIG. 1 schematically illustrates a sectional view of a SOI RF device in conventional technologies. A SOI substrate 1 includes a high resistivity silicon plate 2, a buried oxide layer 3 on the high resistivity silicon plate 2 and a top silicon layer 4 on the buried oxide layer 3, wherein a Shallow Trench Isolation (STI) structure 5 is formed in the top silicon layer 4 to separate active regions in the top silicon layer 4. A semiconductor device, such as a transistor, is formed in the active regions. A metal interconnect structure is formed on the SOI substrate 1. One-layer metal interconnect structure is taken as an example in FIG. 1. The one-layer metal interconnect structure includes the top silicon layer 4, an interlayer dielectric layer 6 on the STI structure 5, a conductive plug formed in the interlayer dielectric layer 6 and a metal layer 7 on the interlayer dielectric layer 6 and the conductive plug. At least a portion of the STI structure 5 is covered by the metal layer 7.
However, in practice, especially in RF applications which require high linearity and low insertion loss, if the above SOI RF device is employed, loss of RF signals may be great and linearity of the RF signals may be poor.